Electrodepositable primer coating compositions, particularly in the automotive industry, are typically corrosion resistant epoxy-based compositions and are typically crosslinked with aromatic isocyanates. If exposed to sunlight, such compositions undergo photodegradation from ultraviolet light. In some automotive applications, a spray applied primer surfacer is applied to the electrocoat before topcoating to provide protection from photodegradation. In other automotive applications, topcoats are applied directly to the electrocoat and in these instances, the topcoat usually prevents UV radiation from reaching the primer surface. However, if a topcoat does not provide sufficient protection, extensive photodegradation of the electrocoat can result in a loose powdery surface being formed on the primer. In such a case, delamination between the primer coat and the topcoat results from the formation of the loose powdery layer.
Typically, if a topcoat is sufficiently opaque to UV light by being filled with pigment and/or light-absorbing compounds, no ultraviolet light penetrates to the surface of epoxy-based primers to cause photodegradation. However, in the case of a thin topcoat and/or a topcoat which is not UV light absorbing, ultraviolet light can pass through the topcoat and cause photodegradation of an epoxy-based primer. The photodegradation results in delamination of the topcoat from the primer coat which produces catastrophic failure of the coating system. This problem is particularly likely to occur when a topcoat is lightly pigmented with metal flake pigments which tend to allow radiation to pass through to the primer.
Typically, the best way to avoid photodegradation of printer coatings is by formulating topcoats to prevent transmission of ultraviolet light to the primer surface. For example, the use of thick topcoats or opaque topcoats is usually sufficient. Also, the use of components in the topcoat to prevent transmission of ultraviolet light, such as ultraviolet light absorbers, can be successful in avoiding photodegradation of the primer. However, reliance on proper formulation of topcoats can be unsatisfactory in the painting operation because of variations in the quality of topcoats. Accordingly, there is a need for a primer composition which retards photodegradation and subsequent delamination independent of the quality of the topcoat.
Other factors can aggravate the photosensitivity of an epoxy-based primer and can contribute to delamination of a topcoat from a primer coat, such as use of aromatic isocyanate crosslinkers or of TiO.sub.2 as a pigment in electrodeposition coatings. Additionally, overbake of the electrodepositable coating at excessive times or temperatures or baking in an oxidizing atmosphere can aggravate the problem.
In copending application Ser. No. 07/745,797 mentioned above, photodegradation-resistant electrodepositable compositions are disclosed. The compositions contain an epoxy-based ionic resin, a hindered amine light stabilizer and optionally a phenolic antioxidant. Although the compositions provide excellent resistance to photodegradation of the electrodeposited primer and excellent resistance to subsequent topcoat delamination, the compositions contain relatively high amounts of hindered amine, typically on the order of about one percent by weight based on resin solids, and low amounts of phenolic antioxidants, i.e., about 0.1 percent by weight. Unfortunately, the hindered amine is a relatively expensive material, particularly in comparison to the phenolic compound, and incorporating the hindered amine into the composition at levels of about one percent can be prohibitively expensive. It would be desirable to find a way to reduce the amount of hindered amine while maintaining optimum resistance to photodegradation of the electrodeposited primer layer and consequent delamination of the subsequently applied topcoat.
U.S. Pat. No. 5,205,916 discloses electrodepositable primer compositions containing an aqueous dispersion of an epoxy-based ionic resin and an antioxidant additive which is a combination of a phenolic antioxidant and a sulfur-containing antioxidant. The additive is disclosed as reducing overbake yellowing of the composition and also reducing intercoat delamination of subsequently applied topcoats upon exterior exposure. However, it has been found that such compositions are not as effective in preventing electrodeposited primer-topcoat delamination as are compositions containing hindered amine light stabilizers.